To ascertain a band gap of the material, there is a need of collecting and analyzing light caused by cathodoluminescence, which occurs when an electron beam is applied to a material. In general, the collection of the light based on the cathodeluminescence is performed inside an electron microscope, and thus the electron microscope includes a main tube mounted with additional elements such as a reflection mirror needed for collecting the cathodoluminescence light.
A conventional electron microscope capable of collecting the cathodoluminescence is relatively bulky and complicated and pretty expensive because of the additional elements. Further, there is a limit that the cathodoluminescence is collectable in only the electron microscope having such a function. However, the conventional electron microscope mounted with the elements for collecting the cathodoluminescence has a structure of collecting the cathodoluminescence generated from one surface of a sample, and therefore obtained information is relatively small in quantity and not accurate.